A discontinuous-mode (DCM) switching power supply generates a regulated output voltage in response to a current that goes to zero for a period of time during each switching cycle.
If the current reverses its direction, i.e., becomes negative or falls below zero, then the efficiency of the power supply may be significantly reduced.
Therefore, a DCM power supply may include circuitry to rectify the current; that is, such circuitry allows the current to flow in a forward direction (i.e., allows a forward current), but inhibits the current from flowing in a reverse direction (i.e., inhibits a reverse current). For example, a DCM power supply may include a transistor in the current path, and may activate the transistor while the current is flowing in the forward direction and deactivate the transistor to block the current from flowing in the reverse direction; that is, the power supply may actively rectify the current by using an active component (here a transistor) to rectify the current instead of using an inactive component such as a diode.
But unfortunately, such rectification circuitry may cause a reduction in the efficiency of a power supply that includes such rectification circuitry. For example, such rectification circuitry may not provide optimum rectification timing, and, therefore, may allow a reverse current to flow for a significant period of time before blocking the reverse current, or may block a forward current for a significant period of time when it should be conducting the forward current.